A porous honeycomb structural body consisting of ceramics having excellent heat-resistance and corrosion resistance is used as a filter for dust-collection use in the various industrial fields, such as chemical, electric power, industrial waste treatment etc., for the purpose of preventing environment pollution, recovering materials from the high temperature gas, and so on. And the porous honeycomb structural body is preferably used as the filter for dust-collection under high temperature and corrosive gas atmosphere, such as a diesel particulate filter (DPF) capturing the particulate matters (PM) exhausted from diesel engines of automotive diesel engines, for example.
As a filter using the porous honeycomb structural body (hereinafter referred to as a honeycomb filter), there may be exemplified a filter of such a structure, in a porous honeycomb structural body 6 having a plurality of cells 4 having been divided and formed by partition walls 2, that the inlet end face B and outlet end face C of the cells are plugged alternately by the plugged portion 8, for example, like the honeycomb filter 1 as shown in the FIG. 1(a) and FIG. 1(b) (here, the numeral 12 is the outer wall as the reinforce member). In case of the honeycomb filter having this kind of structure, it is possible to obtain a treated gas G2 from which the particulate matters in the treating gas G1 have been removed, because the foreign matters are captured by the partition walls 2 when the treating gas G1 is introduced into the cells 4a from the inlet end face B, while the treated gas G2 which penetrates and flows into the adjacent cells 4b is exhausted from the end face C.
The above-mentioned honeycomb structural body is produced by a method comprising, for example, mixing and kneading a green body material containing an aggregate material particle, a pore forming material and an organic binder and so on with a disperse medium to obtain a green body, shaping the green body to obtain a shaped body, then, drying the shaped body to obtain a dried body, furthermore, firing the dried body. In this producing method, there has been a problem that the firing breaking (cracks) may occur in the obtained porous honeycomb structural body not a little.
It is considered that the occurrence of the above-mentioned cracks is caused by burning the inflammables contained in the fired body during the temperature rising process at the time of firing the dried body (the body to be fired), and the inner temperature of the body to be fired rises rapidly. That is, in the firing temperature rising process, the temperature of the outer portion of the body to be fired rises substantially constant speed following with the temperature rising of the firing atmosphere, on the other hand, the temperature of the central portion of the body to be fired rises with higher speed than temperature rising speed of the firing atmosphere by burning of the inflammables such as the pore forming material and the organic binder contained in the body to be fired. Therefore, it is considered that the difference of the temperature of the inner and outer portion of the body to be fired is to be large, and then the cracks may occur in the porous honeycomb structural body by thermal stress.
As the strategy to solve the above mentioned problem, the process, in which the temperature rising speed of the firing atmosphere is strictly controlled so that the difference of the temperature of the inner and outer portion of the body to be fired may not occur during the temperature rising process at the time of firing the body to be fired, is heretofore employed conventionally. The applicant also has already proposed the method for producing a porous ceramics structural body in which the burning of the organic binder progresses as gentle as possible under the firing atmosphere in which the temperature rising speed is controlled low (that is, rising temperature gradually) at the temperature zone in which the organic binder burns (about 180-300° C.), and the method in which the temperature of the firing atmosphere is risen synchronizing with the temperature of the central portion of the body to be fired within the predetermined temperature range (See, for example, Patent Document 1, Patent Document 2).    Patent Document 1: Japanese Patent No. 2543565    Patent Document 2: JP-A-2003-212672